This application relates to and cross-references U.S. patent application Ser. No. 09/843,196 entitled xe2x80x9cMETHOD AND SYSTEM FOR OPERATING TWO OR MORE DYNAMIC ELEMENT MATCHING (DEM) COMPONENTS WITH DIFFERENT POWER SUPPLIES FOR A DELTA-SIGMA MODULATOR OF AN ANALOG-TO-DIGITAL CONVERTERxe2x80x9d which has been contemporaneously filed on the same date as the present application. The present patent application also cross-references pending U.S. patent application Ser. No. 09/591,124 filed on Jun. 9, 2000 to Allen et al. entitled xe2x80x9cVOLTAGE LEVEL SHIFTERxe2x80x9d (hereafter referred to as xe2x80x9cAllen et al. patent applicationxe2x80x9d), which has been assigned to CIRRUS LOGIC, INC., Austin, Tex. The Allen et al. patent application is incorporated by reference herein in its entirety.
1. Technical Field
The present invention relates in general to an analog-to-digital converter, and, in particular, to a modulator for an analog-to-digital (xe2x80x9cA/Dxe2x80x9d) converter. Still more particularly, the present invention relates to a method and system for operating two or more integrator amplifiers with different power supplies for an A/D delta-sigma modulator.
2. Description of the Related Art
Analog-to-digital converters (xe2x80x9cADCsxe2x80x9d) are used to convert an analog signal to a digital signal for digital processing and/or storage. A delta-sigma modulator operates to digitize an analog input signal for an analog-to-digital converter. A delta-sigma modulator for an analog-to-digital (xe2x80x9cA/Dxe2x80x9d) converter generally includes at least an integrator, a summation circuit, and a quantizer coupled together. The integrator performs integration operations on the input signal while the summation circuit adds the integrated signals from the integrator. Some delta-sigma modulators have multiple integrators, which provide multiple stages of integration. The quantizer operates to quantify the added outputs from the summation circuit to provide a digitized signal.
Ideal ADCs digitize the signals without adding any noise. However, due to inherent noise of electrical circuits, the electrical circuits of ADCs add at least some amount of noise (e.g., predominantly including but not limited to thermal noise) to the signals. The dynamic range generally measures the performance of an ADC. The dynamic range is the ratio of the power of the largest signal, which the ADC can digitize to the power of the noise (e.g., xe2x80x9cnoise powerxe2x80x9d) added by the ADC. A desired goal in the design of ADCs is to increase the dynamic range. Ways to accomplish increasing the dynamic range is to reduce the noise power and/or increase the signal power.
In reducing noise power, thermal noise is examined. Thermal noise power is proportional to 2kT/(C*OSR), in which k is the Boltzmann""s constant, T is the temperature, C is the sampling capacitance, and OSR is the oversampling ratio. For example, if the sampling capacitance or oversampling ratio is doubled, then a reduction of three (3) decibels (dB) in the noise power occurs. In order to achieve a twelve (12) dB reduction in the noise power, the capacitor size to oversampling ratio has to be sixteen (16) times larger. The approach of reducing noise power in this manner results in the disadvantages of having to significantly increase the capacitor size and capacitor area. The cost of the chip would significantly increase if the capacitor size and area would have to be increased.
In increasing signal power, the signal power is analyzed. The signal power is proportional to (Vrms)2, in which Vrms is the root mean square (xe2x80x9crmsxe2x80x9d) voltage of the input signal. For example, the rms voltage only needs to be increased by a factor four (4) in order to achieve a 12dB increase in the dynamic range. Furthermore, the analog circuits that are xe2x80x9cupstreamxe2x80x9d in the ADC do not more easily limit the dynamic range of the system. However, the increase of the signal power results in some problems. First, in order to accommodate the larger signal voltages, the circuits must be fabricated from high voltage integrated circuit devices, such as transistors, resistors, capacitors, etc., which are all able to handle high voltages. High voltage circuit devices are generally much physically larger in size and occupation of area than low voltage circuit devices. For example, high voltage transistors may occupy ten (10) to twenty (20) times the physical area compared with low voltage transistors. Secondly, the high voltage circuits consume a more significant amount of power than respective low voltage circuits. For instance, a high voltage circuit with an 18 volt power supply may consume over five (5) times as much power as a low voltage circuit having a 3.3 volt power supply.
Typically, a single power supply drives all components of a delta-sigma modulator. For example, the same power supply would drive the integrator amplifiers of all of the integrators in a delta-sigma modulator. Modulator technology has developed such that a large power voltage, such as 5 volts or higher, is able to drive the modulator. The large power voltage allows the A/D converter to receive and process analog input signals in a wide voltage range, which results in the A/D converter having a wider dynamic range and a higher signal-to-noise ratio. However, physically large components, such as large-sized transistors, are needed for the A/D converter to operate under the large voltage. The use of physically large components makes the size and cost of the A/D converter chip respectively larger and higher. Also, since a large power voltage drives the modulator components, then the A/D converter consumes greater overall power.
The present invention recognizes the need to maintain a large power voltage driving the delta-sigma modulator so that at least a wider dynamic range and a higher signal-to-noise ratio is provided for the A/D converter. At the same time, however, the present invention recognizes the continual need and desire to maintain or reduce the overall physical size, cost, and power consumption of an A/D converter chip.
A method and system are disclosed for operating two or more integrator amplifiers with different power supplies for a modulator of an analog-to-digital (xe2x80x9cA/Dxe2x80x9d) converter. A first, upstream integrator is operated with one power supply, and the other downstream integrator(s) is/are operated with at least another power supply. The modulator has amplifiers with coefficient gains having values that are determined and set so that voltage levels for the at least another integrator are maintained within operating and output limits. An integrating coefficient gain k1 for the first integrator is set to have a sufficiently large value so that an integrating capacitor can be made small for the one integrator. Another integrating coefficient gain k2 for a second integrator is set to have a sufficiently small value so that an output voltage from the first integrator is sufficiently attenuated to a voltage value within an operating range of the second integrator.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.